Method to determine if a variable already exists or not

There is a defined? keyword that performs this function.

#defined?

defined? expression tests whether or not expression refers to anything recognizable (literal object, local variable that has been initialized, method name visible from the current scope, etc.). The return value is nil if the expression cannot be resolved. Otherwise, the return value provides information about the expression.

Test whether the given expression is recognizable or not. If the expression cannot be resolved, it returns nil, otherwise it returns information related to the expression.

>> a = 1
 => 1
>> defined? a
 => "locdal-variable"
>> defined? b
 => nil
>> defined? nil
 => "nil"
>> defined? String
 => "constant"
>> defined? 1
 => "expression"